The transmission of information by carrier currents on home electrical distribution circuits is at the moment instilling profound interest in the present development of home installations.
In a communication system by carrier currents, such as can at the moment be conceived, items of electrical equipment are connected to a mains electrical distribution circuit and can either be intended for receiving commands, or dedicated to the emission of information or of commands intended for items of receiving equipment. The information transmitted is generally digital information resulting from an encoding and from a conditioning of a set of information including one or more addresses, data and monitoring information, the whole constituting a frame which is injected onto the mains by frequency modulation or else amplitude modulation in accordance with well-known techniques. With the present methods of transmission numerous difficulties emerge when several items of emitter equipment are connected up to the electrical distribution circuit at several points of the circuit. Indeed, without special care, the simultaneous emission of information by two emitters or more leads to a collision of the injected frames and to a distortion of the transmitted messages, which may possibly induce inopportune commands and in many cases lead to an absence of correct transmission of information. Thus, the overall reliability of the communication method is severely affected because of the potential risk of collision of information.
In the area of the management of information transmission on a communication medium, the principle is already known of the token ring widespread in local microcomputer networks. With this principle, at any moment, a single microcomputer has the option of emitting information on the network intended for one or more of the other microcomputers connected to the network, and the "token" passes to the next microcomputer in the network, which becomes the new master of the transmission of information on the network. There is thus a collision management of a deterministic nature, which, however, has the disadvantage of leading to a relatively modest information flow by comparison with the limit flows allowed by the usual transmission speeds for this type of transmission.
In the area of communication networks, there also exist methods of bit-by-bit collision detection generally leading to states of arbitration between emitters in the course of which the emitters involved in the collision cease emitting until the network is freed, the emitters moreover being subject to a predetermined order of priority.
These methods of bit-by-bit collision detection which have the disadvantage of not optimizing the flow of information by systematically neutralizing the emission in the event of detection of a collision are not applicable in the case of a transmission within an electrical distribution circuit.
British Patent GB-2 229023A, filed in the name of Creda Limited, dated Mar. 6, 1989, discloses a system for remote control of electrical apparatus by carrier currents on an electrical distribution circuit, this system comprising a master processor and a bus controller which emit data packets towards remote applications. A method for avoiding the information collisions is proposed, the main characteristic being the sending by each appliance having received a data packet emitted by the processor of an acknowledgment of receipt after a predetermined time-span. This method, however, does not implement a bit-by-bit collision detection and does not guarantee that a defective message will be detected by the network.
European Patent EP-A-0 100 594, filed in the name of the Plessey Overseas Company, Ltd., dated Jun. 15, 1983, describes a local area network computer system provided with an anti-collision transmission management employing a priority signal consisting of either a binary number indicative of a measurement of the standby time of a station connected to the network, or of a number allocated to the station for this purpose, or of a combination of these two numbers. This type of collision management does not envision listening for the echo returned by the destination station. Furthermore, such a system cannot be applied to the area of carrier currents.